1. Field of the Invention
The present invention relates to a stacked piezoelectric element in which a piezoelectric layer and an electrode layer are stacked, a manufacturing method thereof and a vibration wave driving apparatus.
2. Description of the Related Art
Conventionally, piezoelectric material is a typical material that has electromechanical energy conversion functions, and is used as piezoelectric elements in various applications. Recently, stacked piezoelectric elements in which plurality of layers are stacked, integrally formed and sintered are commonly used.
For instance, Japanese Laid-Open Patent Publication (Kokai) No. 6-77550, Japanese Laid-Open Patent Publication (Kokai) No. 6-120580 and Japanese Laid-Open Patent Publication (Kokai) No. 8-213664 disclose a configuration in which the stacked piezoelectric element is used for a vibration wave motor as a vibration wave driving apparatus, and in particular, for a vibration wave motor which is formed like a column. Japanese Patent No. 2961545 discloses a vibration wave motor which uses no stacked piezoelectric element but uses a cylindrical and single-layer piezoelectric element and generates a bending vibration as a vibrating body.
The stacked piezoelectric element is characterized in that, in comparison with a single-layer piezoelectric element, a great deformation strain and a great force can be obtained at a low voltage by layer stacking, miniaturization is possible because thickness of one stacked layer can be rendered thinner, and the like.
The stacked piezoelectric element comprises a plurality of piezoelectric layers of piezoelectric material and a plurality of electrode layers of conductive material arranged adjacent to the each piezoelectric layer. A method of manufacturing the stacked piezoelectric element is as follows. In a sheet manufacturing process, a green sheet to be the piezoelectric layer is created from piezoelectric material powder and an organic binder by a method such as a doctor blade method or a die coater so as to render it as an electrode layer by printing a conductive paste in a predetermined position on the green sheet. In a layer stacking process, a predetermined number of the green sheets are stacked on each other's flat surface and stacked by pressurizing them with a press. Thereafter, firing and polarization processes are performed so that they are eventually machined to be plate-formed or ring-formed. The stacked piezoelectric element is manufactured through these processes.
FIGS. 9A and 9B are perspective views showing the stacked piezoelectric element used for the vibrating body of a conventional columnar vibration wave motor. FIG. 9A shows the stacked piezoelectric element in which a plurality of layers are stacked on each other's flat surface, and FIG. 9B shows the stacked piezoelectric element broken down into individual layers.
A stacked piezoelectric element 30 includes a plurality of hollow piezoelectric layers 32 which are stacked on each other's flat surface. The piezoelectric layer 32 has an electrode layer 33 which is quartered (shaded areas in the drawing) formed on its surface. The piezoelectric layer 32 also has connection electrodes 33a formed on its surface, which are connected to the electrode layer 33 and extended to an outer circumference of the piezoelectric layer 32.
The connection electrode 33a connected to each piece of the electrode layer 33 is provided in the same phase position (angular position) on every other layer. The stacked piezoelectric element 30 has interlayer electrodes 34 formed on its outer circumference, which conduct a plurality of connection electrodes 33a located in the same phase positions. The piezoelectric layer 32 positioned as a top layer also has a plurality of surface electrodes 35 formed in a circumferential direction on its surface rim, which are connected to the interlayer electrodes 34 corresponding to them respectively. A voltage is applied to the surface electrodes 35 and the polarization process is thereby performed so as to allow the columnar vibration wave motor which comprises the connection electrode 33a to drive.
Here, a principle of the columnar vibration wave motor is shown. The vibrating body is configured by tightly holding the stacked piezoelectric element with metal components. If a predetermined AC voltage is applied to the stacked piezoelectric element from a driving circuit (not shown), the vibrating body has two bending vibrations orthogonal to its axial direction generated with a temporal phase difference. The two bending vibrations generate an oscillating movement at the end of the metal component. Further, the end of the metal component, generating an oscillating movement, operates as a driving unit which rotates a contact body, pressurizing and contacting the end of the metal component, by means of frictional contact.
FIG. 10 is a sectional view showing a structure of the columnar vibration wave motor including the vibrating body using the stacked piezoelectric element of FIGS. 9A and 9B. This columnar vibration wave motor 40 has a vibrating body 41 incorporated therein. The vibrating body 41 has the stacked piezoelectric element 30 placed between hollow metal components 43 and 44 together with a hollow wiring substrate 42 of which base material is a polymer material so that the wiring substrate 42 contacts the surface electrodes 35 of the stacked piezoelectric element 30. And a bolt 45 inserted from the metal component 43 side is screwed into the metal component 44, and the stacked piezoelectric element 30 is thereby tightly held and fixed between the metal components 43 and 44 together with the wiring substrate 42.
Thus, the AC voltage from the driving circuit is applied to the surface electrodes 35 of the stacked piezoelectric element 30 via the wiring substrate 42. As a result of this, a rotor (contact body) 48 pressurizing and contacting the end of the metal component 44 by means of a spring 46 and a spring support 47 is rotated by the aforementioned oscillating movement so as to take out a rotation output via a gear 49 which rotates together with the rotor 48.
However, the conventional stacked piezoelectric element had the following problem. To be more precise, the vibration wave motor is currently desired to further reduce its manufacturing cost, not to mention realizing miniaturization and higher output thereof.
In the case of miniaturizing the conventional columnar vibration wave motor, however, adhesiveness of a contact surface between the stacked piezoelectric element 30 and the metal component 44 is reduced and influence of periodic damping becomes stronger. Furthermore, this reduction in adhesiveness and the periodic damping cause lowering performance of the vibration wave motor.
In the case of the conventional manufacturing method, large-scaled manufacturing equipment needs for the sheet manufacturing process of manufacturing sheets from the piezoelectric material powder and the layer stacking process of pressing and stacking them, which also increases an amount of capital investment. In addition, machining was eventually required for the piezoelectric elements manufactured from the sheets, which may cause reducing yields of the material as well and also increases the manufacturing cost.
As for the vibration wave motor which uses the cylindrical and single-layer piezoelectric element and generates the bending vibration as a vibrating body, a high voltage was required in the case of increasing a displacement of the bending vibration because of the single layer. For this reason, the cost of electrical parts such as a transformer became high. These parts required large space as an electrical circuit portion in spite of miniaturization.